This invention relates in general to programmable logic controllers and, more particularly, to an apparatus and method for mapping the components of a programmable logic controller computer system to one another while providing local processing capability at the I/O module level.
Programmable logic controllers (PLC's) are a relatively recent development in process control technology. As a part of process control, a PLC is used to monitor input signals from a variety of input points (input sensors) which report events and conditions occurring in a controlled process. For example, a PLC can monitor such input conditions as motor speed, temperature, pressure, volumetric flow and the like. A control program is stored in a memory within the PLC to instruct the PLC what actions to take upon encountering particular input signals or conditions. In response to these input signals provided by input sensors, the PLC derives and generates output signals which are transmitted via PLC output points to various output devices, such as actuators and relays, to control the process. For example, the PLC issues output signals to speed up or slow down a conveyer, rotate the arm of a robot, open or close a relay, raise or lower temperature as well as many other possible control functions too numerous to list.
The input and output points referred to above are typically associated with input modules and output modules, respectively. Input modules and output modules are collectively referred to as I/O modules herein. Those skilled in the art alternatively refer to such I/O modules as I/O cards or I/O boards. These I/O modules are typically pluggable into respective slots located on a backplane board in the PLC. The slots are coupled together by a main bus which couples any I/O modules plugged into the slots to a central processing unit (CPU). The CPU itself can be located on a card which is pluggable into a dedicated slot on the backplane of the PLC.
FIG. 1 shows one typical conventional programmable logic controller system as system 10. System 10 includes a host programmable logic controller 15 coupled by a field bus 20 such as the GENIUS Bus (available from GE Fanuc) to a bus interface unit 25. Bus interface unit 25 couples and interfaces field bus 20 to a local bus 30 which includes a plurality of I/O terminal blocks 35. I/O terminal blocks 35 are coupled to respective I/O modules 40 as shown in FIG. 1.
It is noted that in system 10, all computational processing is performed by the host programmable logic controller 15. In other words conditions are sensed at I/O modules 40 and input data is derived therefrom. The input data is transferred through bus interface unit 25 and field bus 20 to host programmable logic controller 15. Host programmable logic controller 15 acts on the input data according to a control program stored in host PLC 15. Host programmable logic controller 15 processes the input data and produces output data in response thereto. The output data is transferred through field bus 20, bus interface unit 25, local bus 30 to one or more I/O modules 40. In response to the output data, the I/O module receiving the output data controls an output device coupled to the I/O module. In this particular arrangement, no local processing is performed at the I/O module level, but rather all processing is performed by the host PLC.